1. Field of the Invention
The preferred embodiments of the present invention relate, inter alia, to a secondary battery excellent in discharge rate and preferably used as a high power demand battery, especially as an alkaline storage battery.
2. Description of the Related Art
The following description sets forth the inventors' knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art. Recently, with respect to compact second batteries, the market tends to strongly demand initially low prices and subsequently, e.g., high energy density and high power. Electrodes of such secondary battery can be generally classified into sintered-type electrodes and paste-type electrodes. In the case of Ni/Cd batteries, Ni/MH batteries and lithium secondary batteries, a paste-type electrode low in cost, light in weight and high in energy density is preferably used as both the positive and negative electrodes. The paste-type electrode is manufactured typically by impregnating paste containing active and/or pseudo-active material powder as a main ingredient in an electrode substrate, or applying and then drying such paste on the electrode substrate. Typical battery structures are shown in FIGS. 1 and 2. The example shown in FIG. 1 employs a spirally wound electrode unit 7 in which a positive electrode 1 and a negative electrode 2 are wound in a spiral manner with a separator 3 interposed therebetween. The example shown in FIG. 2 employs a laminated electrode unit 7 in which a plurality of positive and negative electrodes 1 and 2 are laminated with separators 3 interposed therebetween. In such batteries, inter alia, sealed cylindrical batteries employing a spirally wound electrode unit, in most cases, it is configured such that a single electrode lead is connected to a positive electrode and a negative electrode is exposed from the external periphery of the electrode unit so as to be in direct contact with a metal battery case. Even if a paste-type electrode is employed as both the electrodes, since the lower end of the separator is forcibly bent to cover the entire lower surface of the electrode unit when the electrode unit is inserted into a battery case, an occurrence of short between the positive electrode and the battery case (i.e., the negative electrode terminal) can be prevented. In cases where high power is required, it is necessary to configure such that end portions of both the electrodes are exposed so as to be in contact with collector plates disposed at both ends of the electrode unit. In this case, however, it was difficult to prevent an occurrence of short at the lower end of the electrode unit.
In a sealed cylindrical battery employing a spirally wound electrode unit, which is popular among secondary batteries, in cases where a pasted type electrode is employed as a positive electrode and a negative electrode, powder of active material or pseudo-active material which absorbs/discharges the active material (hereinafter, both materials may be referred to as “active material”) of the electrodes will be repeatedly expanded and contracted, thereby causing the powder to shed and accumulate between the end portion of the electrode and the collector plate of opposite electrode. The increase of the shedding materials after charge/discharge cycles causes electric short circuit (i.e., short) thereof.
To avoid this problem, for example, it can be considered to configure such that a top and/or bottom end of the spirally wound electrode unit is sealed so as to prevent the shed active material from coming into contacting with the opposite electrode terminal (collector plate). However, such sealing of both the top and bottom ends of the electrode unit causes a limited contactable area between the electrode and the electrode terminal, resulting in unfavorable configuration for high-energy density and high-power demand secondary battery.
Under the circumstances, a sintered-type positive electrode in which active material is hard to be shed has been used for a power use alkaline storage batteries of the cylindrical. (see, e.g., a non-patent document entitled “Foamed Nickel Positive Electrode for A High Performance Cylindrical Ni—Cd Battery Power Sources 12, 203, (1988),” and a non-patent document entitled “Metal Hydride Electrode for High Energy Density Sealed Nickel-Metal Hydride Battery Power Sources 12, 393 (1988).” However, an employment of such a sintered-type positive electrode causes an increased manufacturing cost and an increased weight.
Furthermore, for example, it also can be considered to configure such that an end portion of the electrode unit is bent so as to prevent the shed active material from coming into contact with the electrodes. However, since a punching metal with an average thickness of 60 μm or more is generally used as an electrode substrate, it is difficult to bend the end portion. Even if the end portion of the electrode can be forcibly bent nearly at right angle, the end portion strongly presses a corner of the adjacent electrode, which may cause micro short via a separator at the pressing portion.
The aforementioned problems reside not only in sealed cylindrical batteries employing a spirally wound electrode unit but also in lamination type batteries as shown, for example, in FIG. 2.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. Indeed, certain features of the invention may be capable of overcoming certain disadvantages, while still retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.